The invention as far as the submarine is concerned has it's basis in the Norwegian patent application 850957 and represents a futher development of the concept described therein, especially with the intention to enable operations to be carried out optionally by any interaction with conventional techniques performed from a location above sea level (floating platform or vessel) thereby taking full advantage of developed, well-known and proven techniques and systems for subsea field development, operation and maintenance, making feasible full utilization of the advantages of a submarine concept (direct access-visually and diverless), while preserving the opportunity to take a step forward into a desirable completely non-surface, technological phase, a development which has promising perspectives in connection with offshore production of oil and gas, in particular at considerable depths.
Underwater production systems usually include the utilization of a so-called template or foundation frame, with room for several wellheads and associated equipment. Such a template can for instance comprise eight wellheads, advantageously arranged in two parallel rows, with four wellheads in each row. The underwater production system is advantageously constructed of units or modules. These modules are mounted and removed by means of guide posts and corresponding guide funnels, arranged in a standard system For each module, four guide posts are employed, one positioned in each corner of a square of standard dimensions. The modules can advantageously have guide funnels arranged in the same configuration, for guiding co-operation with the guide posts. During development of a field, i.e. installation of the underwater production system, modules are brought down and retrieved, from a location such as a rig, or a vessel on the surface. Guidelines are employed which run from the respective guide posts mounted in the foundation frame, up to the vessel. These guidelines can be fastened to the guide posts and reach up to a buoy when not in use, the guide lines being fished up by the surface vessel when it is necessary to install or replace one or several modules. The guide lines can also be releasably connected to the guide posts. For attaching of guidelines, divers or remotely controlled mini-submarines are used. The securing and connecting of the modules requires the work of divers, or the use of remotely controlled mini-submarines, or the use of complicated hydraulic remotely controlled tools.
During operation and maintenance the same technique is employed for the control and replacement of modules.
Installation and maintenance of an underwater production system, comprises not only the lowering and lifting up of modules, but also requires other kinds of operations, in particular coupling and uncoupling of pipelines are mentioned here. Such coupling and uncoupling can also be undertaken by means of work modules which are transported down from a rig or a vessel, utilizing guide posts and guide funnels.
The conventional technique indicated above is well known and proven and functionally quite satisfactorily. An important disadvantage however is its surface dependence and consequently its strong dependence on weather conditions at the surface. Continuously increasing depths makes it difficult or not very desirable to use divers. Remotely controlled mini-submarines have also been seen to have limited use.
In the initially mentioned Norwegian patent application 850957 (U.S. Pat. application Ser. No. 131,337 filed Dec. 9, 1987, which was a continuation of U.S. Pat. application Ser. No. 838,118 filed March 10, 1986 which claimed the priority of the Norwegiam patent application 850957) there is therefore suggested an apparatus for the execution of operations under water, comprising a twin hull submarine which can be combined with a number of defined cargo modules, where the twin hull submarine is provided with a cargo holder between it's two hulls, for carrying of cargo modules in the compartment between the hulls. With this kind of apparatus surface dependence can be avoided, i.e. it will be possible to install, run and maintain underwater production systems independent of surface weather conditions such as high seas, strong wind, drifting ice etc. The submarine is autonomous, that is, it is self reliant and manned and travels out from a suitable base on the coast, where loading, unloading, maintenance and bunkering take place. This means that work can take place in a system which to the greatest possible extent remains operational under water the whole time (travel out--work--travel to base). Independence of surface weather conditions then relates not only to the execution of work, but also during travelling to and from the place of work.
As mentioned, the conventional technique, with the use of guide lines, guide poles and guide funnels, is well proven and recognised. There is therefore a need for continuing use of this recognised technical concept, at the same time having the possibility to abolish surface dependence. In the said Norwegian patent applicatin 850957 a surface-independent system is outlined, without describing in detail how the apparatus can be utilized in combination with already known surface-dependent technique.
In addition to the guide line technique mentioned above, lowering and lifting-up of modules from a surface vessel without the use of guide lines is also known, whereby drive units (propel units) either mounted onto, or attached to the modules, are employed, enabling the module to be steered into position above the intended location, where the guide posts/funnel arrangements are still preferably used. The steering in and mounting naturally take place by means of divers, underwater cameras, mini-submarines, various types of navigation equipment etc.
Underwater production systems of the kind described above, will usually be surrounded by a protective structure. This is suitably constructed as a powerful frame welded together of pipe elements and placed around the production system. In some wates, for instance in the North Sea, it is necessary for such a protective structure to be overtrawlable, i.e. it should be designed so that fishing trawls will not become caught up in it, in the case of over-trawling. The present invention demands the use of such a protective structure, overtrawlable or not.